Pressure builds in radiator

[Xulfiqar]

Thanks for ur detailed reply.i know a little bit about the cooling principle and also the circulation theory.

About the heater core query, i have its inlet and outlet pipes blocked so this option will be deleted (heater was working previously,i will update about it in a day or two)
Now coming back to ur confusion about the problem.
Cap was opened when guage was at mid (assuming that guage is working perfectly) water came out.problem is that if i open cap when engine is cold i.e in morning, i feel some pressure being released but this time water does not come out.
Cold engine at startup, small bubbles seen.At this point i have a confusion.As u said that the long circulation starts after reaching specified temp indicated on valve, then how can bubbles be seen in cold engine??as the water stands stationary until valve has opened.This does not obiviously apply to my case as i have removed t-valve.

Once again thnkx @msabbasi.

That sound you hear when you open the radiator cap on a stone cold system is the sound of air filling the remaining little vacuum in the system that could not be replenished by the expanded coolant due to the spring pressure of the vacuum valve in the cap, Its normal. If you had not opened the cap and driven off the vacuum would have been replaced with water within the radiator as it starts to expand.

If you want to confirm any residual pressure in the system then try to squeeze the radiator rubber pipes, they will be really soft and easy to squeeze. If you do that on a normal hot engine the pipes will be under about 14 psi pressure (depending on cap rating) and the pipes would be hard.